Among the dynamic focusing electron guns, there is one which has a quadrupole lens as shown in FIG. 1. Said the electron gun comprises a prepositioned triode part consisting of cathode K, control grid G1 and screen grid G2, and a main lens system consisting of focusing electrode E1, dynamic electrode E2 and anode E3 which respectively receive a static focusing voltage, a dynamic focusing voltage, and a static anode high voltage.
On the beam exiting side of said focusing electrode E1 and on the beam entrance side of said dynamic electrode E2, vertically elongate beam passing holes H1 and laterally elongate beam passing holes H2 respectively are disposed facing each other to form a quadrupole lens. On the beam exiting side of said electrode E2 and on the beam entrance side of said anode E3, elongate common beam passing holes H2' and H3 are disposed. Isolating metal ribs 12, 12 and I3, I3 are vertically attached to the exit and entrance sides of the electrode E2 and the anode E3, respectively, to form three separate beam passing zones in holes H2' and H3.
Besides, in a conventional dynamic focusing electron gun constructed as described above, the dynamic electrode receives a parabolic dynamic focusing voltage which is synchronized with the horizontal and vertical scanning signals according to the scanning position of electron beam. The dynamic focusing voltage is applied to the dynamic electrode in addition to a static potential focusing voltage in such a manner that, when the electron beam lands on the central part of the screen, a dynamic voltage Vd of OV or low positive potential is applied, and when the electron beam lands on the periphery of screen, a high potential dynamic voltage is applied. Therefore, whether or not a quadripole dynamic lens is formed between the focusing electrode E1 and the dynamic electrode E2 is determined by the landing position of the electron beam. Thus, when a quadrupole lens is formed when the electron beam is scanned on the periphery of the screen, the electron beam becomes vertically elongated by the asymmetrical dynamic electric field formed by the vertically elongate beam passing holes H1,H1,H1 and laterally elongate beam passing holes H2,H2,H2.
The vertically elongated electron beam scanned toward the periphery of the screen passes through the deflection yoke for rectification of the distortion of the beam by a non-homogeneous magnetic field, with the result that a nearly complete circular beam spot is formed on the screen. Moreover, when the electron beam lands on the periphery of the screen, said dynamic voltage rises and thereby the strength of final acceleration and focusing lens formed between the dynamic electrode and the anode becomes weaker.
Thus, the focal distance of the electron beam becomes longer so that the focus of the electron beam is formed on the periphery of the screen which is farther from the electron gun than the central part of the screen and the beam spot formed on the screen becomes very small, thereby realizing high resolution of image.
However, there are shortcomings in the conventional dynamic focusing electron gun in that it is difficult to realize a voltage controlling device which can endure the high parabolic dynamic voltage, the static focusing, voltage must be applied to the dynamic electrode, and an arc may occur at the neck of the cathode ray tube by the leakage of current due to the high voltage. Further, the conventional dynamic focusing electron gun has two focusing electrodes, and therefore the magnification of the major lens which finally accelerates and focuses the electron beam must be increased. Thus, there is a fear that the quality of image will become worse because of spherical aberration due to the high magnification of the major lens.